feat(ui): path-based app-router routes with 404 and project auth preview

Resolve routes from pathname (and legacy hash), add a notFound route with
a dedicated 404 page, and show a blurred workspace preview for
unauthenticated project visits. Wire up Next.js app-router entry files and
listen for popstate so history navigation updates the route.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/infrastructure/designGateway.ts

@@ -168,19 +168,17 @@ export const designGateway = {
 
   agentSocket(projectId: string, payload: AgentChatPayload, handlers: ProjectEventHandlers) {
     let closed = false;
-    let closeEvents: (() => void) | null = payload.threadId ? designGateway.projectEvents(projectId, handlers, { threadId: payload.threadId }) : null;
+    let closeEvents: (() => void) | null = null;
 
     void designGateway
       .agentChat(projectId, payload)
       .then((response) => {
         if (closed) return;
         handlers.onProject?.(response.project);
-        if (!closeEvents) {
-          closeEvents = designGateway.projectEvents(response.project.id, handlers, {
-            threadId: response.threadId || payload.threadId,
-            eventsUrl: response.eventsUrl
-          });
-        }
+        closeEvents = designGateway.projectEvents(response.project.id, handlers, {
+          threadId: response.threadId || payload.threadId,
+          eventsUrl: response.eventsUrl
+        });
       })
       .catch((error: Error) => {
         if (!closed) {